In current common-rail type direct injection systems, a low-pressure pump supplies fuel from a tank to a high-pressure pump, which in turn supplies the fuel to a common rail. A series of injectors (one for each cylinder of the engine) is connected to the common rail, said injectors being driven cyclically in order to inject part of the pressurised fuel present in the common rail into a respective cylinder. If the injection system is to operate correctly, it is important for the fuel pressure level within the common rail constantly to be maintained at a desired value that generally varies over time; to this end, the high-pressure pump is dimensioned so as to supply the common rail in any operating state with a quantity of fuel that exceeds actual consumption and a pressure regulator is coupled to the common rail, which regulator maintains the fuel pressure level within the common rail at the desired value by discharging excess fuel to a recirculation channel that reintroduces said excess fuel upstream from the low-pressure pump.
Known injection systems of the type described above have various disadvantages, because the high-pressure pump must be dimensioned so as to supply the common rail with a quantity of fuel that slightly exceeds the maximum possible consumption; however, this maximum possible consumption state occurs relatively rarely and in all other operating states the quantity of fuel supplied to the common rail by the high-pressure pump is much greater than that actually consumed and thus a considerable proportion of said fuel must be discharged by the pressure regulator into the recirculation channel. Obviously, the work performed by the high-pressure pump in pumping fuel that is subsequently discharged by the pressure regulator is “pointless” work and such known injection systems accordingly have very low energy efficiency. Moreover, such known injection systems have a tendency to overheat the fuel, because when the excess fuel is discharged by the pressure regulator into recirculation channel, said fuel passes from a very high pressure (greater than 1000 bar) to a substantially ambient pressure and this pressure drop tends to increase the temperature of the fuel. Finally, known injection systems of the type described above are relatively bulky owing to the presence of the pressure regulator and the recirculation channel connected to the pressure regulator.
In order to overcome the problems described above, a solution has been proposed of the type presented in patent application EP0481964A1, which describes the use of a high-pressure pump with a variable flow rate, which is capable of supplying the common rail with only the quantity of fuel that is necessary to maintain the fuel pressure within the common rail at the desired value; in particular, the high-pressure pump is equipped with an electromagnetic actuator capable of instantaneously varying the flow rate of the high-pressure pump by varying the closure time of an intake valve of the high-pressure pump itself.
Another embodiment of a high-pressure pump with a variable flow rate is described by patent U.S. Pat. No. 6,116,870A1. In particular, the high-pressure pump described by U.S. Pat. No. 6,116,870A1 comprises a cylinder provided with a piston that has reciprocating motion within the cylinder, an intake channel, a delivery channel coupled to the common rail, an intake valve capable of permitting fuel to flow into the cylinder, a non-return delivery valve coupled to the delivery channel and capable only of permitting fuel to flow out of the cylinder, and a regulating device coupled to the intake valve in order to keep the intake valve open during a compression phase of the piston and so permit the fuel to flow out of the cylinder through the intake channel. The intake valve comprises a valve body that is mobile along the intake channel and a valve seat, which is capable of being acted upon in a fluid-tight manner by the valve body and is located at the opposite end of the intake channel from the end communicating with the cylinder. The regulating device comprises an actuating body, which is coupled to the valve body and can move between a passive position, in which it permits the valve body to act in a fluid-tight manner upon the valve seat, and an active position, in which it does not permit the valve body to act in a fluid-tight manner upon the valve seat; the actuating body is coupled to an electromagnetic actuator, which is capable of displacing the actuating body between the passive position and the active position.
As stated above, in the above-described high-pressure pumps with a variable flow rate, the flow rate of a high-pressure pump is varied by varying the closure time of the intake valve of said high-pressure pump; in particular, the flow rate is reduced by delaying the closure time of the intake valve and is increased by advancing the closure time of the intake valve.
In general, the above-described high-pressure pumps with a variable flow rate have two cylinders, along each of which there runs a piston that completes one cycle (i.e. performs an intake stroke and a pumping stroke) for every two revolutions of the drive shaft; thus, for every two complete revolutions of the drive shaft, the high-pressure pump makes two pump strokes (one for each cylinder of the high-pressure pump). In a four-cylinder, four-stroke internal combustion engine, for each complete revolution of the drive shaft, one pump stroke of the high-pressure pump and the injection phase of two injectors take place. When the flow rate required is equal or close to the maximum flow rate of the pump, both the injectors performing the injection phase during any one revolution of the drive shaft inject the fuel while a piston of the high-pressure pump is pumping the fuel into the common rail; when the required flow rate is less than the maximum flow rate of the high-pressure pump, the pump stroke is choked and thus a first one of the injectors performing the injection phase during any one revolution of the drive shaft injects the fuel while neither piston of the high-pressure pump is pumping fuel into the common rail, whereas a second one of the injectors performing the injection phase during any one revolution of the drive shaft injects the fuel while one piston of the high-pressure pump is pumping fuel into the common rail. The disparity described above, which arises between the two injectors performing the injection phase during the same revolution of the drive shaft results in a disparity in the quantity of fuel injected by the two injectors with an identical injection time, with obvious repercussions on the correct operation of the engine; moreover, this disparity does not always occur to the same extent, but there is a substantial difference when the flow rate required from the high-pressure pump is lower than a certain threshold value corresponding to the value at which choking of the high-pressure pump coincides with the beginning of the injection phase of the first injector to inject, out of the two injectors performing the injection phase during the same revolution of the drive shaft.
In order to overcome the above-described disadvantage, at least in part, it has been proposed to use a high-pressure pump with a variable flow rate having two cylinders, along each of which there runs a piston that completes one cycle (i.e. performs an intake stroke and a pumping stroke) for each revolution of the drive shaft. Thus, in a four-cylinder, four-stroke internal combustion engine, for each complete revolution of the drive shaft, two pump strokes of the high-pressure pump and the injection phase of two injectors take place; in this manner, just one injection phase of one of the injectors always takes place during each pump stroke of the high-pressure pump. When the required flow rate is equal or close to the maximum flow rate of the pump, all the injectors inject the fuel while one piston of the high-pressure pump is pumping fuel into the common rail; when the required flow rate is less than the maximum flow rate of the high-pressure pump, the pump stroke is choked and all the injectors inject the fuel while neither piston of the high-pressure pump is pumping fuel into the common rail. Obviously, the disparity in the behaviour of the injectors is reduced because, within any one control interval, either all the injectors perform injection while one piston of the high-pressure pump is pumping fuel into the common rail, or all the injectors perform injection while neither piston of the high-pressure pump is pumping fuel into the common rail; nevertheless, a slight disparity in behaviour remains in that in some control intervals the injectors have certain dynamic characteristics because they are injecting while one piston of the high-pressure pump is pumping fuel into the common rail, whereas in other control intervals the injectors have different dynamic characteristics because they are injecting while neither piston of the high-pressure pump is pumping fuel into the common rail.
Moreover, making the pistons of the high-pressure pump perform one cycle (i.e. an intake stroke and a pumping stroke) on each revolution of the drive shaft instead of one cycle every two revolutions of the drive shaft entails doubling the average velocity of said pistons with obvious problems of mechanical strength and reliability over time. Alternatively, it has been proposed to use high-pressure pumps equipped with four cylinders and thus with four pistons, each of which performs one cycle every two revolutions of the drive shaft; however, while this solution is more straightforward to implement, it involves substantially higher costs and bulkiness of the high-pressure pump.
EP0962650A1 discloses an accumulator-type fuel injection apparatus having a plurality of fuel injection valves for corresponding individual cylinders of an engine; the fuel injection valves are connected to a common pressure-accumulator chamber that is connected to an ejection side of a fuel pump. Fuel is pumped from the fuel pump into the pressure-accumulator chamber and then supplied into the cylinders via the corresponding fuel injection valves; the fuel pumping timing of the fuel pump is set relative to the fuel injection timing so that a variation in fuel pressure in the pressure-accumulating chamber at the time of start of a fuel injecting operation is smaller than a predetermined set value.
EP1130250A1 discloses a pump having a housing with working chamber, reciprocally moving piston rotatably mounted about its longitudinal axis and at least one inlet opening; opening in piston casing is connected to working chamber, interacts with inlet opening and is designed so liquid flowing into working chamber can be adjusted to turn the piston, which has radial groove with radial depth of at least one per cent of piston diameter. The pump has a pump housing with a working chamber, a reciprocally moving piston rotatably mounted about its longitudinal axis and at least one inlet opening; an opening in the piston casing is connected to the working chamber, interacts with the inlet opening and is designed so the liquid flowing into the working chamber can be adjusted to turn the piston. A groove extending along the periphery of the piston has a radial depth amounting to at least one per cent of the piston diameter.
EP0501459A1 discloses a common-rail fuel injection system for an engine including a common rail for storing fuel; a plurality of pumps supply fuel to the common rail. Fuel is injected into the engine from the common rail and feedback control is executed on the pressure of the fuel in the common rail; a device serves to detect whether or not at least one of the pumps fails and an arrangement decreases the pressure of the fuel in the common rail when the detecting device detects that at least one of the pumps fails.
EP1241338A1 discloses a fuel supply system, which reduces the unevenness of injection rates of cylinders in a fuel supply system of a direct injection engine which uses a variable displacement single plunger pump; the unevenness of injection rates of cylinders can be reduced by constructing so that the cam which drives the high-pressure fuel pump may make one reciprocation while the engine makes explosions by two cylinders and causing the controller to extend the injection time width of one of two injectors which inject while one discharge of the high-pressure fuel pump and to shorten the injection time width of the other injector.